The present invention relates generally to methods for extruding metals and alloys and more particularly to an extrusion method for high temperature metallic and intermetallic alloys.
A large number of metallic and intermetallic materials for high temperature application to aircraft propulsion and airframe systems may be formed to semifinished or finished shapes via conventional deformation processes only with great difficulty. In conventional extrusion, a billet is preheated in a high temperature furnace or induction unit, placed into tooling which is heated to only relatively low temperatures (typically &lt;500.degree. F.) and then extruded through a die. Conventional extrusion practice for a high temperature metallic or intermetallic alloy also usually includes sealing it in a sacrificial can, which protects the alloy against oxidation during furnace or reduction preheating prior to extrusion and shields the alloy from die chilling as the billet-can assembly is extruded. Without the can, the billet may cool to a regime of poor workability and fail during the extrusion process.
To provide the desired protection against chilling, the can must not thin excessively or tear during extrusion and thus allow the billet to contact the die. The can material must therefore flow at about the same rate as the billet, that is, the deformation resistance (flow stress) of the can material should approximately equal that of the billet. Unfortunately, for most common inexpensive can materials (e.g., stainless steels, conventional titanium alloys), the flow stress (at the same temperature as the billet) is substantially less. In this case, the billet will move like a nearly rigid body into the soft can during the initial stages of extrusion, pinch the can wall and contact the die, and lead to partial or catastrophic failure.
The invention solves or substantially reduces in critical importance problems with conventional processes, by combining special processing parameters, can geometry and design of the interface layer between billet and can. By these means, a temperature differential between can and billet is established immediately prior to extrusion. Can temperature is reduced so that its flow stress is about equal to that of the hotter billet within, which enhances uniformity of can and billet flow.
A wide variety of metals and alloys, including difficult-to-work high temperature alloys, may be extruded using the method of the invention, to produce finished or semifinished products of a variety of extruded shapes, with an overall product yield approaching 100% of the starting material.
It is therefore a principal object of the invention to provide an improved extrusion process.
It is a further object of the invention to provide an extrusion process for metals and alloys, particularly difficult-to-work high temperature alloys.
It is yet another object of the invention to provide an extrusion process for difficult-to-work high temperature alloys utilizing inexpensive can materials.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.